THIS invention relates to a spark gap protection device which is used to protect a circuit from a transient or surge overvoltage or current.
Various spark gap devices are used in lightning protection devices, commonly known as surge protective devices. For example, in a lightning protection device gas-type spark gaps are used to protect an electrical circuit against transient overvoltages.
Gas-type spark gaps are electrical elements which normally have a very high electrical resistance, which can be considered infinite (e.g. an open circuit). When this type of spark gap instantaneously breaks down or trips under a transient high voltage, the spark gap triggers and conducts current at a very low resistance which is similar to a short circuit. The spark gap breaks down when the overvoltage exceeds the trip voltage of the spark gap and conducts a discharge current, caused by the transient overvoltage, to earth. The spark gap device thereby protects a downstream circuit against any transient overvoltages above the threshold value of the trip voltage.
For example, electrical circuits operating at 220V may use a spark gap protection device with a trip voltage of 500V. This trip voltage would typically be higher than the normal operating voltage, but below the voltage that would be hazardous to the electrical circuit to be protected.
Spark gaps, and in particular the gas-type spark gaps, also have a surge current rating which is dependent on the construction of the spark gaps. The surge current rating indicates the current level above which the spark gap cannot conduct currents without being damaged or destructed. As transient overvoltages occur randomly with varying intensities, spark gaps are subjected to erosion and damage that cannot be predicted. Although it is possible to construct spark gap protection devices with high enough current ratings to withstand high overvoltages and surge currents and to alleviate some of the effects of erosion and damage, such devices are bulky, complex and expensive to manufacture.
U.S. Pat. No. 4,267,484 shows a parallel multi-electrode spark gap switch used to switch high peak currents. The multiple electrodes extend from a peripheral edge of a main electrode and are aligned with corresponding electrodes on a facing electrode. When subjected to a trigger, one pair of electrodes will break over and conduct before the other electrodes. However, as this happens, the current build up in the pair of electrodes will cause a flux time rate of change in the high permeability cores of ferrite material that surround the electrodes. By transformer action, these cores will cause the remaining aligned pairs of electrodes to increase in potential difference and to break down one by one, until all are conducting, thereby sharing the surge current.
US Patent Application Publication No. 2004/0070914 shows a lightning arrester device for protecting an electrical circuit connected to a low-voltage network against transient overvoltages. This document discloses a lightning arrestor device that has a plurality of gas-type spark gaps which individually has a surge current rating below a desired surge current rating of the device. The spark gaps may be designed to trip simultaneously in parallel by controlling the construction of the spark gaps. In this prior art document a set of varistors is connected in series with the spark gaps, and one or more thermal disconnects are arranged between the varistors and the spark gaps so that the thermal disconnect will disconnect when the varistor heats excessively as a result of surge current reaching excessive levels. Visual indication of such failure may also be provided.
A problem that has been identified with some of the prior art spark gap protection devices is that not all the spark gaps which may be connected in parallel would trigger. For example, where parallel spark gaps are used, a surge voltage may appear across the terminals of the spark gaps, which will cause the spark gap with the lowest spark-over voltage to trigger first.
As explained, this spark gap becomes a short circuit, causing all parallel spark gaps to have a low voltage (almost zero volts) across their terminals. The voltage across the parallel combination is accordingly not sufficient for the spark gaps which have not flashed over to trigger. The one spark gap that has sparked over will conduct the entire surge current. As the surge current may exceed the rating of the spark gap, it may be damaged because the surge current rating of the individual spark gap is lower than the total desired rating of the parallel combination of spark gaps.
In some instances, there may be a probability that the conducting spark gap may ionize the surrounding spark gaps, allowing them to flash over, but this process does not guarantee repeatability, i.e. that other spark gaps will be ionized for every surge current conducted. These types of spark gap protection devices may accordingly not have the required reliability.
The main drawbacks of the existing spark gap protection devices where parallel firing is achieved are that specialised spark gaps are needed for the construction of the protection devices, as “off the shelf” spark gaps cannot be used and that the manufacture of these protection devices necessitates custom machining, which increases the cost of manufacturing the protective devices.
It is an object of the invention to provide an alternative spark gap protection device.